Set accelerators are used when it is necessary to work with hydraulic cement compositions at freezing or near-freezing temperatures. The rate of hydration of Portland cement, for example, is very dependent on temperature; Portland cement compositions will often harden at a rate slower than desired unless the hardening process is accelerated. In addition to accelerating the set, it is desirable to also attain an increase in the rate of early strength development at normal or low temperatures. This reduces curing and protection periods necessary to achieve specified strengths in concrete. Some techniques for accelerating the set are: increasing the proportion of cement in the mix, heating the components used in the mix, and using chemical admixtures that act on the components of the mix to increase the rate at which the cement paste sets.
Several chemical set accelerators are well-known. They include alkali hydroxides, silicates, fluorosilicates, calcium formate, sodium chloride, calcium chloride, and calcium nitrate and calcium nitrite.
Calcium chloride is widely used because it is easy and inexpensive to produce; its effects on hydraulic cement are also predictable and well-documented. However, like sodium chloride, it has the disadvantage of corroding steel. Thus, its presence in sufficient amounts can weaken concrete containing steel reinforcing bars which have corroded. Other drawbacks of calcium chloride use include reduced compressive strength at later ages, reduced response to air entrainment, and blotching of hardened concrete surfaces (Dodson, Concrete Admixtures, Van Nostrand Reinhold, 1990).
In light of this there is a continuing need in the art for improved set-accelerating admixtures. In particular, there is a need for stable, safe and inexpensive set accelerators which may be easily admixed into cement compositions, and which are additionally advantageous for use as cold-weather set accelerators.
It is thus an object of this invention to provide a new class of set-accelerating admixtures which possess the above qualities and which are particularly advantageous in reducing the set time of hydraulic cement compositions at reduced temperatures. Further objects of the invention are to provide cement compositions containing the set accelerators of this invention, and to provide methods for reducing the set time of hydraulic cements.